Siwarex U SFC58/SFC59 Zeroing and Span Adjustment on S7-300

1. Overview of SIWAREX U Weighing Integration

The SIWAREX U (order numbers 7MH4950-1AA01 for the single-channel variant and 7MH4950-2AA01 for the dual-channel variant) is a SIMATIC function module (FM) that integrates strain-gauge load cell acquisition directly into a SIMATIC S7-300 CPU or an ET 200M distributed I/O station. The module exposes its parameters, process values, and command interface through Data Records (DS) accessed over the backplane/PROFIBUS-DP using the standard SIMATIC SFCs SFC58 WR_REC (write record) and SFC59 RD_REC (read record).

This reference explains how to use SFC58 and SFC59 to:

• Read gross/net process values from Data Record 32 (DS32, RECNUM B#16#20).
• Trigger the zeroing command (tare/zero set) by writing Data Record 3 (DS3, RECNUM B#16#3) and the command byte within Data Record 12 (DS12, RECNUM B#16#C).
• Trigger the span calibration command and write the adjustment digit 0 / digit 1 values for the calibration weight (Calibration Weight 1 / 2).
• Verify command acceptance by re-reading DS32 and the status bits in DS3 / DS12.

All record layouts and command codes below are taken from the official SIWAREX U Device Manual (Edition 38, 06/2010) and the Siemens Industry Online Support portal.

2. Prerequisites

Before implementing the SFC58 / SFC59 interface, verify the following:

1. SIMATIC S7-300 CPU (e.g. CPU 315-2 DP) with STEP 7 V5.5 or TIA Portal V13+ and the SIWAREX U HSP / GSD file installed.
2. SIWAREX U FM inserted in the S7-300 rack (or ET 200M IM 153) with PROFIBUS address 3..32 assigned in HW Config. The slot must match the configured FM type: 7MH4950-1AA01 (1-channel) or 7MH4950-2AA01 (dual-channel).
3. Load cells wired to the SIWAREX U D-sub connector following the 4-wire / 6-wire sense-line conventions in chapter 3 of the Device Manual. The shield must be terminated on the grounding bar of the cabinet.
4. Supply voltage 24 V DC on the FM and stable 5 V / 6 V excitation to the load cells.
5. Logical base address of the FM in HW Config. This is the input to IOID / LADDR in SFC58 and SFC59 and is required for every call.
6. Data blocks in the user program sized to the record lengths shown in the table below:

3. SFC58 / SFC59 Call Signature

Both SFCs operate on the basis of the PROFIBUS-DP Record service. The relevant input parameters are:

Use IOID = B#16#54 when calling on the input (input-side) base address of the FM, which is the convention for SIWAREX U.

4. Data Record Layout for Reading (DR32)

Data Record 32 returns the user-unit process values for the two channels of the dual-channel module. Per the Device Manual, the record is 10 bytes per channel with the following layout (big-endian, word-aligned):

An example SFC59 call to read channel 2 into DB802:

// ST (SCL) — read process value Ch.2
CALL SFC 59 (
    REQ    := TRUE,             // triggered by OB1 cycle or HMI button
    IOID   := B#16#54,
    LADDR  := W#16#100,         // example: input base address 256
    RECNUM := B#16#20,          // DR32
    RECORD := P#DB802.DBX 0.0 BYTE 10,
    RET_VAL:= MW100,
    BUSY   := M101.0,
    DONE   := M101.1,
    ERROR  := M101.2);

The RECNUM = B#16#20 and RECORD = P#DB802.DBX0.0 BYTE 10 arguments are exactly those referenced in the original integration question — the FM returns one 10-byte block per channel; for the dual-channel variant (7MH4950-2AA01) a second SFC59 call is required to read the second channel.

5. Data Record Layout for Writing Commands (DR12)

DR12 is the command and adjustment interface. The 10-byte layout is:

The SFC58 call to write DR12 is therefore:

// ST (SCL) — write command + adjustment digits
CALL SFC 58 (
    REQ    := M50.0,            // rising edge from HMI button
    IOID   := B#16#54,
    LADDR  := W#16#100,
    RECNUM := B#16#C,           // DR12
    RECORD := P#DB80.DBX 10.0 BYTE 10,
    RET_VAL:= MW102,
    BUSY   := M103.0,
    DONE   := M103.1,
    ERROR  := M103.2);

Where the RECORD ANY pointer P#DB80.DBX10.0 BYTE 10 matches the source integration question: command byte at DB80.DBX10.0, qualifier at DB80.DBX11.0, adjustment digit 0 at DB80.DBD12.0, and adjustment digit 1 at DB80.DBD16.0. Some integrations split digit 0 / digit 1 into adjacent bytes (DBX12.0 / DBX16.0); the word-aligned interpretation above is the one used in the Device Manual and Siemens sample FB FB250 SIW_U.

6. Command Code Reference

The command byte in DR12 is decoded by the FM as follows (decimal values shown with the equivalent hex):

The qualifier byte selects the target channel on the dual-channel module: 0 = both channels, 1 = channel 1 only, 2 = channel 2 only.

7. Step-by-Step: Zeroing from an HMI

1. Pre-load the command record in DB80:

// DB80 layout
DB80.DBX  10.0  BYTE  := 1;       // command 1 = zero set
DB80.DBX  11.0  BYTE  := 0;       // qualifier 0 = both channels (single-channel: 1)
DB80.DBD  12.0  DINT  := 0;       // digit 0 (not used for zero set)
DB80.DBD  16.0  DINT  := 0;       // digit 1 (not used for zero set)

2. Raise the HMI button flag DB80.DBX18.0 (TRUE for one cycle) to trigger SFC58 REQ.
3. Wait for DONE = TRUE and ERROR = FALSE. The zeroing is finished in < 100 ms; poll the command-pending bit in DR12 command byte — it returns to 0 when the FM has accepted the command.
4. Read DR32 again (SFC59 with RECNUM = B#16#20) and confirm the gross weight is now within 1 count of 0.
5. If ERROR = TRUE, store RET_VAL in the HMI diagnostic view and reference the error code table in §10.

8. Step-by-Step: Span Calibration from an HMI

Yes — span adjustment is also written with SFC58, but to a different combination of code and adjustment digit. The span (the kg/count slope) is not written directly; instead the FM is told "command 5 (or 6) and the calibration weight in 0.1 kg / count units". The FM then internally recomputes and stores the new characteristic.

1. Set command code = 5 (Calibration weight 1) or 6 (Calibration weight 2). The example below uses code 5 (single-stage span calibration).
2. Write the actual calibration weight that is currently on the scale (in 0.1 kg) into adjustment digit 0 (DB80.DBD12.0). For a 250.0 kg test weight, write L#2500.
3. Trigger SFC58 exactly as in §7.
4. After DONE = TRUE, re-read DR3 with SFC59 RECNUM = B#16#3 to confirm the calibration valid bit is now set; if it is cleared, the FM rejected the command and the previous calibration remains active.

// Span calibration: code 5, calibration weight 250.0 kg on Ch.1
DB80.DBX  10.0  BYTE  := 5;       // command 5 = calibration weight 1
DB80.DBX  11.0  BYTE  := 1;       // qualifier 1 = channel 1
DB80.DBD  12.0  DINT  := L#2500;  // 250.0 kg expressed in 0.1 kg
DB80.DBD  16.0  DINT  := 0;       // not used for single-stage cal

For a two-stage calibration (recommended by Siemens for installations with a high dead load and a small live load), first issue command 5 with the larger calibration weight (digit 0), then issue command 6 with the smaller calibration weight (digit 1). The FM combines both into the final characteristic curve.

9. Alternative Implementation: Reusable FB (SCL)

The following SCL function block wraps the SFC58 call and is safe to call from a cyclic OB. It includes the rising-edge handling the SFC requires and returns a structured status word.

FUNCTION_BLOCK FB_SIWAREX_CAL
VAR_INPUT
    iLADDR      : WORD;          // FM base address
    iReqZero    : BOOL;          // 1-cycle TRUE = perform zero set
    iReqSpan    : BOOL;          // 1-cycle TRUE = perform span cal
    iCalWeight  : DINT;          // calibration weight in 0.1 kg
    iQualifier  : BYTE;          // 0=both, 1=Ch1, 2=Ch2
END_VAR
VAR_OUTPUT
    oDone       : BOOL;
    oBusy       : BOOL;
    oError      : BOOL;
    oStatus     : WORD;          // SFC RET_VAL
    oCmdEcho    : BYTE;          // last command sent
END_VAR
VAR
    sTrig       : BOOL;          // edge memory
    sBusy       : BOOL;
    sDone       : BOOL;
    sError      : BOOL;
    sRet        : INT;
    sCmd        : BYTE;
    sDB12       : ARRAY[0..9] OF BYTE;
END_VAR
BEGIN
    // Edge detection — issue command only on 0 -> 1 of the HMI flag
    IF (iReqZero OR iReqSpan) AND NOT sTrig THEN
        sTrig := TRUE;
        IF iReqZero THEN
            sCmd := 1;           // zero set
        ELSIF iReqSpan THEN
            sCmd := 5;           // calibration weight 1
        END_IF;
        sDB12[0] := sCmd;        // command
        sDB12[1] := iQualifier;  // channel select
        // pack calibration weight into adjustment digit 0 (bytes 2..5)
        sDB12[2] := BYTE(SHR(IN:=WORD(iCalWeight), N:=24) AND 16#FF);
        sDB12[3] := BYTE(SHR(IN:=WORD(iCalWeight), N:=16) AND 16#FF);
        sDB12[4] := BYTE(SHR(IN:=WORD(iCalWeight), N:= 8) AND 16#FF);
        sDB12[5] := BYTE(                WORD(iCalWeight)        AND 16#FF);
        sDB12[6] := 0; sDB12[7] := 0; sDB12[8] := 0; sDB12[9] := 0;
        sBusy := TRUE;
        CALL SFC 58 (
            REQ    := TRUE,
            IOID   := B#16#54,
            LADDR  := iLADDR,
            RECNUM := B#16#C,
            RECORD := P##sDB12,
            RET_VAL:= sRet,
            BUSY   := sBusy,
            DONE   := sDone,
            ERROR  := sError);
    ELSIF NOT (iReqZero OR iReqSpan) THEN
        sTrig := FALSE;
    END_IF;

    oDone    := sDone;
    oBusy    := sBusy;
    oError   := sError;
    oStatus  := WORD(sRet);
    oCmdEcho := sCmd;
END_FUNCTION_BLOCK

For TIA Portal users, the same logic applies; the FB SIW_U shipped with the SIWAREX U HSP contains pre-built SFC58 / SFC59 wrappers and can be inspected instead of writing a new one.

10. SFC58 / SFC59 Return Value (RET_VAL) Decoding

Common error codes returned in RET_VAL when working with SIWAREX U:

11. Verification Procedure

1. After every zero command, read DR32 with SFC59 RECNUM = B#16#20. The gross weight must read 0 ± 1 count and the "weight stable" bit (DR32 status bit 4) must be 1.
2. After every span command, read DR3 with SFC59 RECNUM = B#16#3. The "calibration valid" bit must be 1 and the "calibration changed" bit must be 0 (i.e. the FM has stored the new characteristic).
3. Place a known test weight on the scale and verify the displayed weight in the HMI matches the test weight within the rated accuracy. For trade-legal installations the permissible error is defined in OIML R76 and must be confirmed by an authorized metrology inspector.
4. Cycle power to the FM and re-read DR3 / DR32. The zero and span values must persist — they are stored in non-volatile memory on the FM.

12. Troubleshooting Matrix

13. Notes on Platform Variants

• STEP 7 V5.5 / V5.6: SFC58 / SFC59 are part of the standard library "Standard Library → Communication Blocks". No additional library is required.
• TIA Portal V13+: SFC58 / SFC59 are inside the "Communication" → "Other" folder. The SIWAREX U HSP also installs the sample FB SIW_U that exposes typed methods cmdZero, cmdSpan, etc., and should be used in preference to hand-written SFC calls where possible.
• ET 200M with PROFIBUS: identical procedure; the FM is a DP slave and the CPU sends the SFC58 / SFC59 records over PROFIBUS-DP using the slot/index mechanism.
• ET 200SP / PROFINET: SIWAREX U is not natively PROFINET-capable. It is supported over PROFINET only when the FM is installed in an ET 200M behind an IM 153-4 PN. The SFC58 / SFC59 call signature does not change.

14. Frequently Asked Questions